A series of volcanic eruptions may have helped bring about the downfall of the last Egyptian dynasty 2,000 years ago.

By suppressing the monsoons that swelled the Nile River each summer, triggering flooding that supported the region’s agriculture, the eruptions probably helped usher in an era of periodic revolts, researchers report online October 17 in Nature Communications. That upheaval ultimately doomed the dynasty that ruled Egypt’s Ptolemaic Kingdom for nearly 300 years until the death of Cleopatra.

To piece together this puzzle, Yale University historian Joseph Manning and his colleagues first compared records of Nile River heights dating back to A.D. 622 with volcanic eruptions recorded in ice cores from Greenland and Antarctica that date back 2,500 years. Ash layers in the ice cores, corresponding to “eruption years”, were linked to years of less extensive flooding, they found.

Then, to see how eruptions would have affected precipitation in Ptolemaic Egypt, the researchers simulated how climate changed after five large 20th century eruptions. Each eruption ultimately reduced rainfall across the regions of Africa that drain into the Nile. Powerful eruptions can wreak havoc on monsoons by shifting and weakening the Intertropical Convergence Zone, a belt of low pressure near the equator that drives nearby precipitation patterns.

Finally, Manning and colleagues pored over historical texts from Ptolemaic Egypt, comparing periods of unrest with the volcanic record in the ice cores. Eruptions coincided with the onset of many recorded revolts. Political instability, famine and drought may have come to a head around 44 B.C., when Italy’s Mount Etna erupted explosively. The Ptolemaic dynasty soon came to a close in 30 B.C. with Cleopatra’s suicide.